Automatic shifting transmissions use fluid operated friction devices, such as clutches and brakes, to establish gear ratios between the vehicle engine and drive wheels. The gear ratios are interchanged (upshifted or downshifted) to extend the usefulness of the engine operating range. During a ratio interchange, it is necessary to control the engagement timing of the on-coming friction device as well as controlling the disengagement point of the off-going friction drive device.
Fluid operated disc type friction devices have a free running clearance. That is, when a device is disengaged, the adjacent plates do not have significant contact to thereby maintain the slip losses of the device at a minimum. Free running clearance affects the engagement timing of the device. The apply piston must be pressurized to move through a distance defined by the free running clearance prior to clutch engagement. The movement should occur as quickly as possible. Thus, a large flow volume is originally required during the engagement process. The flow volume must be rapidly reduced to avoid a harsh apply of the friction device which will affect the ratio interchange.
Engagement timing is generally provided in current transmissions using one of three control methods depending upon the acceptable cost for the transmission. One control system incorporates one-way devices in combination with friction devices. These systems require duplicate arrangements if engine coast braking is to be available. However, the ratio change on and off of one-way devices is known to be quite smooth.
A second of the control systems uses variable pressure control devices, such as pulse-width-modulated valves to control the on-coming and off-going friction devices. These pulse-width-modulated devices require an electronic control module or computer to affect proper control of the variable pressure devices.
The third system uses a fluid accumulator and valve to control the on-coming friction device and a bleed orifice to control the off-going device. The accumulator requires additional space within the transmission housing as well as a valve mechanism that permits the accumulator to be filled when the friction device is engaging and to not interfere with the exhausting of the friction device during disengagement.